Yuna and the player are first introduced to Shuyin in the farplace (introduced as in, you finally get to know his name). She mistakes him for Tidus, and is obviously pisses her off because he keeps metioning a girl named "Lenne". Tidus and Shuyin have some obvious physical differences, but Yuna can't seem to tell them apart because it's been two years since the last time she saw Tidus. Anyway, the reason behind all of this confusion is because of the songstress dressphere. It enables Yuna to wear the same clothes as Lenne did, and Shuyin mistakes Yuna for Lenne (on a number of occasions, actually). The most notable occasion is Yuna waking up to find Shuyin in the Farplane.

Yuna meets Shuyin in the Farplane

Yuna:

What's happening?

???:

I finally found you.

Yuna:

Is that really you?

Shuyin:

It is me, Shuyin. I've waited so long, Lenne.

Yuna:

But I'm not Lenne.

Shuyin:

Lenne. We disappeared together, but when I awoke, I was alone. I looked
for you for so long. While I wandered, I realized something: Spira hasn't really
changed at all. Everyone's still fighting over nothing. Still dying like they
used to. A thousand years have passed, and they can't leave the hatred behind.
I'm through waiting. I'll fix it. This world continues to fail us, and what's
worse, I failed to protect you. (clenches his fist) Vegnagun will make that all
go away. And we'll fade together again, together. Help me do it, Lenne.